1. Field of the Invention
The present invention relates to a bonding structure and a method thereof, especially to a Sn—Ag bonding and a method thereof.
2. Description of Related Art
Along with fast development of technologies, there is a trend in electronic components to be more light, thin and compact. A conventional single material is unable to meet these requirements of component design. Generally, each material has it own properties such as mobility of electron/hole, light absorbance, reflection rate, heat Conductivity, electrical resistance, and mechanical properties etc. However, in practice, there is no single material with optimum effects on various physical properties. Thus in order to achieve best photoelectric/electronic effect of photoelectric/electronic components, the properties of different materials must be integrated.
In early days, various materials are integrated by heteroepitaxial technique or ion implant technique used in manufacturing processes of integrated circuit (IC). The biggest problem encountered by heteroepitaxial technique is lattice match. Once the requirement of lattice match is not satisfied, the high quality epitaxial film can't be obtained and functions of the components are further affected. Moreover, the thickness of the epitaxial film produced by heteroepitaxial technique is no more than 10 μm. Such way is neither efficient nor cost-effective.
Furthermore, once ion implant is used to integrate various materials, a transition layer with high defect density is formed and the functions of components are affected.
Recently, an area of research that integrates various materials has become mature. The technique is wafer bonding technique that allows the integration of materials with lattice mismatch by means of wafer bonding and removal processes. The main purpose of wafer bonding is in building composite materials by bonding different materials and the composite materials with various properties are applied to different fields broadly. There are different types of wafer bonding as listed below (1) direct wafer bonding (2) anodic wafer bonding (3) low temperature wafer bonding (4) intermediate layer wafer bonding (5) adhesive wafer bonding . . . etc. The most common way is direct wafer bonding and adhesive wafer bonding. The direct wafer bonding is a method to join two same or dissimilar materials together while the adhesive wafer bonding is a bonding approach having an intermediate layer for bonding between two wafers.
Direct Wafer Bonding has been widely developed and has become very attractive for a lot of applications. It is also called Van der Waals bonding. Chemical bonds (electric dipole) are formed between two mirror wafers or epitaxial layers with very flat surfaces by chemical solutions. The wafers are initially quick bonded via weak Van der Waals bonding force. Then wafer pairs are applied with pressure and are heated. The wafer cleaning, the pressure applied, the heated temperature and time, and other parameters are determined according to the bonding material. Before heating, the direct bonding relies on weak Van der Waals force. The bonding energy obtained after heat processing is from diffusion of atoms at the interface.
In addition, the adhesive wafer bonding includes an intermediate bonding medium such as metal, wax, epoxy, and SOG (spin-on-glass). Thus annealing temperature and time of wafer bonding are reduced and the produced components are with better properties.
Now wafer bonding technique is broadly applied to photoelectric/electronic industries such as the improvement of performance of photoelectric components, manufacturing and applications of SOI (Silicon-on-insulator) chips, manufacturing and integrations of Si Discrete Power Devices as well as MEMS (Micro Electro Mechanical Systems) devices, and Optoelectronic Integrated Circuits (OEIC) manufactured by integration of photoelectric components and Ultra-Large Scale Integration (ULSI) chips. The above description explains how the wafer bonding technique is applied to optoelectronic components.
In order to use energy efficiently, develop high technology and protect the earth, the high brightness white light LED has become main point of development in solid state lighting in developed countries. It is estimated the light efficiency of high brightness white light LED will achieve 200 lm/W within 15 years so that it will replace all lighting devices in our daily lives at that time. Thus the electricity consumed by lighting equipments is reduced 50% and the overall electricity is saved up to 10%. Moreover, about two hundred million tons of carbon dioxide emitted is reduced. Thus not only energy is saved but also environmental protection is achieved.
The development of GaN based LED dramatically increases possibility of mass production of white light LED and plays a key role on that. Up to present, sapphire has played an important role in the improvement of internal quantum efficiency of GaN LED along with fast development of epitaxial technique and it also has great effect on the external quantum efficiency. In order to make a breakthrough, begin with packaging.
Due to low heat conductivity-40 W/moK, the poor heat dissipation capacity of sapphire severely affects internal quantum efficiency of GaN LED. In recent years, researchers have tried to grow GaN on silicon substrate whose heat dissipation capacity and conductivity are better than those of sapphire.
Besides, the wafer bonding technique can be used. The GaN LED is bonded with substrates having better thermal conductivity by metal bonding. The common bonding structure includes Au—Si bonding, Au—Sn bonding and Au—Ag bonding. The bonding temperature of the Au—Si bonding as well as the Au—Sn bonding is 363 and 282 degrees respectively while bonding temperature of the Au—Ag bonding is low temperature-150 degrees. The present invention provides a Sn—Ag bonding bonded at low temperature and a method thereof that further improves component performance as compared with Au—Ag bonding.